1. Field of the Invention
The present invention relates to a crystalline semiconductor thin film, a method for manufacturing the same, a thin film transistor and a method for manufacturing the same.
2. Discussion of the Background
Liquid crystal displays are characterized by their small thicknesses and light weights, capability of being driven at a low voltage, ease of color display, etc. Recently, they are being used as displays of personal computers, word processors and the like. In particular, so-called active matrix liquid crystal displays having a thin film transistor (TFT) as a switching element at each pixel provide a display system, which is presently most suitable for full color televisions and displays for office automation. The reason is that they are not subject to any reduction in contrast, response and the like, even when they have a great number of pixels, and that they are capable of display in halftones.
Such an active matrix liquid crystal display has a substrate configuration formed by two flat glass substrates, i.e., an array substrate and a counter substrate, and a liquid crystal layer sandwiched by the substrates. A color filter and a transparent electrode (counter electrode) associated with each pixel are formed on one of the glass substrates (counter substrate). Pixel electrodes constituted by transparent electrodes and TFTs connected to respective pixel electrodes at the source electrodes thereof are provided on the other glass substrate (array substrate). The gate electrodes of the TFTs are connected to address lines, and their drain electrodes are connected to data lines provided in a direction perpendicular to the address lines.
Address signals are applied to the address lines, and data signals are applied to the data lines. A data signal can be applied to each pixel electrode by applying an address signal at predetermined timing.
The orientation or light transmittance of the liquid crystal layer can be controlled by a potential difference between the counter electrode and pixel electrode, which makes it possible to present any desired display. Details of such liquid crystal displays are described in an article by T. P. Brody et al. (IEEE Trans. on Electron. Devices, Vol. ED-20, November, 1973, pp. 995-1001).
Amorphous Si, polycrystalline Si, etc have been used as semiconductor materials for TFTs. In an active matrix liquid crystal display utilizing polycrystalline Si, drive circuits for applying drive signals to the gate lines and data lines can be formed on the same substrate. This is advantageous in that the display panel can be made compact and in that high reliability of wiring connections can be achieved. The use of such built-in drive circuits makes it possible not only to cause a TFT active matrix liquid crystal display to perform a simple display function as a display but also to turn it into a high performance display having various input/output functions, arithmetic functions, image processing functions and the like.
The characteristics of a built-in drive circuit significantly depend on the characteristics of TFTs. The speed and performance of a circuit becomes higher, the higher the performance of the TFTs.
FIG. 27 is a plan view of a conventional polycrystalline Si thin film showing a top surface thereof. FIG. 28 is a sectional view taken along the line a-axe2x80x2 in FIG. 27. A polycrystalline Si thin film 283 is formed on an insulated circuit 281 with an insulation film 282 made of SiO2 or the like interposed therebetween. A glass substrate is primarily used as the insulated substrate 281. The polycrystalline Si thin film 283 is formed by microscopic crystalline regions 271 referred to as xe2x80x9ccrystal grainsxe2x80x9d. Since each of the crystalline regions 271 has a different crystal orientation, a grain boundary 272 which is a surface of discontinuity of crystals is formed between grains.
Methods for manufacturing the polycrystalline Si thin film 283 include a method in which an amorphous Si thin film is crystallized in a solid phase through a thermal process and a method in which such a thin film is irradiated with an excimer laser to be melted and crystallized.
However, a TFT utilizing a polycrystalline Si thin film has mobility lower than that of a MOSFET which is formed directly on a single crystal Si substrate and therefore provides circuit performance lower than that of a circuit utilizing a MOSFET.
The reason is that polycrystalline Si is formed by an aggregation of small crystal grains unlike a single crystal. Polycrystalline Si has no crystal continuity at each grain boundary where crystal grains are contiguous to each other. Therefore, although it has solid state properties similar to those of single crystal Si in each of the crystal grains, it includes many defects as a polycrystalline film. A polycrystalline Si thin film has defects at grain boundaries because the orientation of Si in each Si crystal grain is not controlled and the crystal orientation of each crystal grain is different.
This has resulted in a problem in that a TFT formed using a polycrystalline Si thin film has low mobility. There has been another problem in that the characteristic of a transition from an off state to an on state, i.e., slope characteristic (S factor) is not so steep.
One method for providing a Si thin film having excellent crystallinity is shown in FIG. 29.
First, an insulation thin film 292 made of SiO2 or the like is formed on a single crystal Si substrate 291 and is provided with holes 293 in part thereof, and an amorphous Si thin film 294 is then formed. Thereafter, a thermal process is performed to promote crystallization from the regions of the amorphous Si thin film 294 in contact with the single crystal Si substrate 291 in the holes 293 which serve as nuclei.
Although this method provides excellent crystallinity, it can not be applied to liquid crystal displays, image sensors and the like in which the substrate must be translucent because a thin film is formed on a single crystal Si substrate 291. Further, presently available single crystal Si substrate can not be used in electronic apparatuses with a large area because the maximum size of them is on the order of 8 inches (200 mm) in terms of the longer span thereof.
Another method for providing a single crystal Si thin film is to bond a single crystal Si substrate directly on to a quartz substrate and to thin the single crystal Si substrate by polishing and etching the same. However, it is quite difficult to form a single crystal Si substrate with a uniform thickness on the order of 100 nm using this method. It has another problem in that device characteristics can be adversely effected by non-uniformity of characteristics at the bonding interface.
As described above, conventional Si semiconductor thin films formed on an insulated substrate have had a problem in that they provide TFT characteristics significantly lower than those of MOSFETs made of single crystal Si because of the use of amorphous Si or polycrystalline Si.
In the case of the method in which an insulation film having holes are formed on a single crystal Si substrate and an amorphous Si thin film formed on the same is crystallized, problems have arisen in that translucence can not be achieved and in that a thin film with a large area can not be obtained.
In the case of the method in which single crystal Si is bonded on to an insulated substrate and is thinned down, problems have arisen in that the bonding to the substrate can be non-uniform and unstable and in that it is quite difficult to achieve, for example, a desired Si film thickness with high accuracy and uniformity when the Si substrate is etched after the bonding.
As described above, presently available TFTs have problems with characteristics such as mobility, and it has been difficult to provide high speed circuits, high precision analog circuits and the like when they are used in liquid crystal displays and the like.
The above disadvantages of conventional systems may be overcome, and other objects may also be accomplished with the present invention, the first embodiment of which provides a method for manufacturing a semiconductor thin film, which includes:
preparing a single crystal semiconductor substrate, said single crystal semiconductor substrate including:
at least one catalytic metal on a surface thereof; and
a semiconductor element;
forming a non-crystalline semiconductor thin film on a second substrate, said non-crystalline semiconductor thin film including said semiconductor element;
touching said surface of said single crystal semiconductor substrate to said semiconductor thin film; and
crystallizing said semiconductor thin film at a temperature lower than an inherent crystallizing temperature of a pure semiconductor layer that includes said semiconductor element.
Another embodiment of the present invention provides a method for manufacturing a semiconductor thin film, which includes:
preparing a single crystal semiconductor substrate, said single crystal semiconductor substrate including:
at least one catalytic metal on a surface thereof; and
a semiconductor element;
forming a non-crystalline semiconductor thin film on a second substrate, said non-crystalline semiconductor thin film including said semiconductor element;
touching said surface of said single crystal semiconductor substrate to said semiconductor thin film; and
crystallizing said semiconductor thin film by low temperature solid state epitaxy.
Another embodiment of the invention provides a method for manufacturing a semiconductor thin film, the method including:
preparing a single crystal semiconductor substrate, said single crystal semiconductor substrate including:
at least one catalytic metal on a surface thereof; and
a semiconductor element;
forming a non-crystalline semiconductor thin film on a second substrate, said non-crystalline semiconductor thin film including said semiconductor element;
touching said surface of said single crystal semiconductor substrate to said semiconductor thin film; and
crystallizing said semiconductor thin film; wherein
after said crystallizing, said semiconductor thin film has a deviation between crystal orientations of adjoining crystallized regions of less than 5 degrees.
Another embodiment of the invention provides a method for manufacturing a semiconductor thin film, the method including:
preparing a single crystal semiconductor substrate, said single crystal semiconductor substrate including:
at least one catalytic metal on a surface thereof; and
a semiconductor element;
forming a non-crystalline semiconductor thin film on a second substrate, said non-crystalline semiconductor thin film including said semiconductor element;
touching said surface of said single crystal semiconductor substrate to said semiconductor thin film, said semiconductor thin film having a larger area than said surface of said single crystal semiconductor substrate;
touching said surface of said single crystal semiconductor substrate to said semiconductor thin film; and
crystallizing said semiconductor thin film at a temperature lower than an inherent crystallizing temperature of a pure semiconductor layer that includes said semiconductor element.
Another embodiment of the invention provides a method for manufacturing a semiconductor thin film transistor including a channel, a gate, a source and a drain, the method including:
preparing a single crystal semiconductor substrate, said single crystal semiconductor substrate including:
at least one catalytic metal on a surface thereof; and
a semiconductor element;
forming a non-crystalline semiconductor thin film on a second substrate, said non-crystalline semiconductor thin film including said semiconductor element;
touching said surface of said single crystal semiconductor substrate to said semiconductor thin film;
crystallizing said semiconductor thin film at a temperature lower than an inherent crystallizing temperature of a pure semiconductor layer that includes said semiconductor element; and
patterning said semiconductor thin film for said channel.